The present disclosure relates generally to optical sensors, and more particularly, to optical seekers, such as used for guiding a steerable rocket or other projectile.
In a projectile application, optical seekers are typically located at the nose or tip of the projectile, such as in a laser-guided projectile. Conventional laser-guided projectiles typically have a silicon quadrant detector forming an illuminated-spot locating detector (ISLD). This quadrant detector has four light-detecting elements, generally of equal size, that have silicon light-absorbing regions and is often referred to as a 4-quadrant detector. In this conventional 4-quadrant detector, the output currents from the four elements are processed in an analog manner to obtain the location of the centroid for the spot of light.
Conventional 4-quadrant detectors that are typically fabricated in silicon have strong absorption for light of wavelengths between 0.48 and 1.06 μm. In order to sense eye-safe wavelengths, 4-quadrant detectors have been made that use InGaAs light-absorbing material. However the InGaAs detectors have a fairly high capacitance (e.g., 1.3×10−4 pF/μm2).
To achieve angle of arrival (AOA) sensing with a large angular field of view while using a lens with a reasonable F-number (e.g., F/1 and larger), it is desirable for the overall active-area of the detector to be larger than 15 mm on a side and, in some cases, larger than 25-30 mm on a side. Also, it is desirable to achieve that large active area while providing a pulse-response rise time that is a factor of 5 to 10 shorter than that provided by the conventional 4-quadrant detectors.
One approach to keeping the large overall active detector area from limiting the response speed or measurement bandwidth is to use an array of photodiode (PD) elements. This PD array groups the elements into sets that occupy four quadrants of the array such that the outputs from the elements in each quadrant are summed together, with the summed signal being output from the array. Prior to being summed, the photocurrent signal from each PD element is amplified and then is passed through a thresholding circuit that blocks or eliminates the inputs having a magnitude below a threshold value. This thresholding prevents the summed noise output obtained when no pulse of light is illuminating the array from being falsely interpreted as additional pulses. However, this thresholding circuit is not able to reduce the noise that is added to the magnitude of the photocurrent pulse when such a pulse is present. In addition to the summed signal for the four quadrants, this known array provides the outputs of the individual array elements in a serially multiplexed fashion, on a frame by frame basis. Thus, the output rate for the signals from individual array elements is quite slow (<<100 kIHz).
In some systems, additional post-processing circuitry is included to track the moving location of the spot of light. Such systems combine the location-determination linearity of an array having many elements with the sub-spot interpolation achieved with the inherently non-linear, but high spatial-resolution 4-quadrant approach. Also, instead of using a PD array as an effective 4-quadrant detector, the outputs from the array elements also may be used to compute the centroid (or the center of gravity, or the moment) of the illuminating spot. Such centroid determination typically is performed using an image processing computer or a field-programmable gate array (FPGA). The error in determining the location of the center of the spot is reduced as the illuminated spot is made wider. However, the numerous analog arithmetic operations of multiplying, summing, and dividing involved in calculating the centroid requires substantial signal processing resources for a sensor that needs to determine the centroid location in real time.
In some systems, signal processing load may be reduced by using an array of binary detectors. However, this array is intended for frame-by-frame readout. Moreover, the input to the thresholding circuit includes the contributions of both the photo-generated current and the noise currents.
Thus, PD arrays have been provided with a binary output to achieve a compact AOA sensor that has wide field of view. However, in order to reduce the location-estimation error of the associated binary centroid determination method, the number of elements in the array that are illuminated concurrently by a given spot of light has to be increased. As a result, the higher accuracy binary centroid location requires the illuminating light to be quite intense, so that the corresponding energy can be spread over many PD elements of the array and still have the signal-to-noise ratio for each array element be sufficiently high.
Thus, it is desirable to provide a centroid-locating AOA sensor that can provide high-resolution and high-accuracy estimation of the incidence angle of a beam of light collected by the aperture of the sensor even for lower levels of the intensity of that light. Moreover, the conventional systems do not provide a high-linearity, centroid-locating PD-array AOA sensor that can process the outputs of the PD elements directly, in a manner similar to that of the 4-quadrant detectors, and that does not require the signals from the PD array elements to first be read out in a serially-multiplexed, frame-by-frame manner before the determination of the centroid location is performed.
Moreover, while it is desirable to enlarge the size of the illuminated spot on a binary PD array in order to improve the centroid location accuracy, the higher sensitivity for incident light of weaker intensity is obtained by spreading that incident light over the smaller area of a smaller illuminated spot on the PD array. Thus, there is a trade-off between achieving high centroid-location accuracy and high sensitivity.